1. Field of the Invention
The present invention relates to an encapsulated toner for heat-and-pressure fixing used for development of electrostatic latent images in electrophotography, an electrostatic printing, or electrostatic recording, and to a method for production of such an encapsulated toner.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer, subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As indicated above, the toner must meet the requirements not only of the development process, but also of the transfer process and the fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, deteriorates after copying from several thousands to several ten thousands of sheets. Such deterioration of the toner can be prevented by using a tough resin having such a high molecular weight that it can withstand the above mechanical frictional forces. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method, such as oven fixing or radiant fixing with infrared rays, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method, such as a heat-and-pressure fixing method using a heat roller, which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device, curling of the paper and an increase in energy consumption. Furthermore, the resin described above is poor in grindability, thereby remarkably lowering the production efficiency of the toner. Accordingly, the binding resin having too high of a degree of polymerization and also too high of a softening point cannot be used.
Meanwhile, according to the heat-and-pressure fixing method using a heat roller, the surface of a heat roller contacts the surface of a visible image formed on an image-receiving sheet under pressure, so that the thermal efficiency is excellent and therefore widely used in various high-speed and low-speed copying machines. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller is coated with a material having excellent release properties for the toner, such as a fluororesin, and further a releasing agent such as silicone oil is applied thereon. However, the method of applying a silicone oil necessitates a larger-scale fixing device, which is not only expensive but also complicated, which in turn may undesirably cause various problems.
Although processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been disclosed in Japanese Patent Examined Publication No. 493/1982 and Japanese Patent Laid-Open Nos. 44836/1975 and 37353/1982, the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature wherein high-temperature offsetting occurs as much as possible, the serviceable fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of curling of paper.
From the above reasons, the development of a toner excellent in fixing ability and offset resistance has always been desired.
A method has been proposed for achieving the low-temperature fixing by using an encapsulated toner comprising a core material and a shell formed thereon so as to cover the surface of the core material.
Among such toners, those having a core material made of a low-melting wax which is easily plastically deformable, as described in U.S. Pat. No. 3,269,626, Japanese Patent Examined Publication Nos. 15876/1971 and 9880/1969, and Japanese Patent Laid-Open Nos. 75032/1973 and 75033/1973, are poor in fixing strength and therefore can be used only in limited fields, although they can be fixed only by pressure.
Further, with respect to toners having a liquid core material, when the strength of the shell is low, the toners tend to break in the developing device and stain the inside thereof, though they can be fixed only by pressure. On the other hand, when the strength of the shell is high, a higher pressure is necessitated in order to break the capsule, thereby giving images that are too glossy. Thus, it has been difficult to control the strength of the shell.
Further, there has been proposed, as a toner for heat-and-pressure fixing, an encapsulated toner for heat roller fixing which comprises a core material made of a resin having a low glass transition temperature which serves to enhance the fixing strength, though blocking at a high temperature may take place if used alone, and a shell of a high-melting point resin wall which is formed by interfacial polymerization for the purpose of imparting blocking resistance to the toner. However, in Japanese Patent Laid-Open No. 56352/1986, this toner cannot fully exhibit the performance of the core material, because the melting point of the shell material is too high and also the shell is too tough and not easily breakable. On the same line of thinking as that described above, encapsulated toners for heat roller fixing with an improved fixing strength of the core material have been proposed (see Japanese Patent Laid-Open Nos. 205162/1983, 205163/1983, 128357/1988, 128358/1988, 128359/1988, 128360/1988, 128361/1988 and 128362/1988). However, since these toners are prepared by a spray drying method, a higher load to the equipments for the production thereof becomes necessary. In addition, they cannot fully exhibit the performance of the core material, because they have not come up with a solution for the problems in the shell.
Further, in the encapsulated toner proposed in Japanese Patent Laid-Open No. 281168/1988, the shell is made of a thermotropic liquid crystal polyester, and in the encapsulated toner proposed in Japanese Patent Laid-Open No. 184358/1992, a crystalline polyester is used. Since each of the polyesters used in these references is not amorphous, the resin melts sharply. However, the amount of energy required for fusion is large. Further, Tg of the core material is also high, thereby making the fixing ability of the resulting toner poor.
Also, as for methods for encapsulation proposed in Japanese Patent Laid-Open No. 128357/1988, there are mentioned such methods as an immersion method using a solvent, a spray-drying method and a fluidizing bed method, all of which have problems in that they require complicated operations.
Further, there have been attempts to control the chargeability of the encapsulated toner in the presence of a charge control agent in the shell of the encapsulated toner or on the surface of the encapsulated toner. However, in the developing process, the charge control agent becomes detached from the toner due to friction with the carrier and adheres onto the carrier. As a result, triboelectric charge of the resulting toner is lowered, thereby causing such problems as background contamination and scattering of the toner in the developer device. In addition, when no charge control agents are present on the surface of the toner, the charging speed may become slow depending upon the type of carriers, thereby causing background contamination, or scattering of the toner in the case of quick printing.